Conventionally, for recording information on a master optical disk, a blue or ultraviolet laser has been used as a recording beam, which is focused on a substrate disk coated with a photosensitive material, so that the substrate disk is exposed to the laser. At this time, the recording beam is focused by an objective lens with a relatively large numerical aperture of 0.9, for example. In this case, the depth of focus is small, and therefore, the objective lens is moved in accordance with the displacement of a surface of the substrate disk so that the focus always is located on the surface of the substrate disk.
Because of this, a system for detecting the displacement of the surface of the substrate disk is indispensable. For example, in addition to the recording beam, a long-wavelength light to which a photosensitive material is not sensitive is radiated through the same objective lens, and based on the reflected light of the long-wavelength light, the displacement of the surface of the substrate disk is detected by a focus detecting method such as an astigmatism method and a skew method. Using a feedback control that moves the position of the objective lens with a detection signal obtained by the displacement detection, an automatic focus control is performed so as to allow the focus to follow the displacement of the surface of the substrate disk.
Further, in recent years, as a recording density of optical disks becomes higher, the possibility of using an electron beam as a recording beam is being considered. However, it is difficult structurally to radiate light other than the electron beam on the surface of the substrate disk through a lens for the electron beam in the conventional manner, and accordingly, another displacement detecting means is necessary.
To this effect, an optical lever method has been developed as shown in FIG. 6, in which a substrate disk surface 403 is irradiated with oblique light, and the displacement of the substrate disk surface 403 is detected from a change in the position at which the reflected light of the oblique light is incident upon a position detector surface 402. According to this method, the light from a light source 401 is reflected by the substrate disk surface 403, and the reflected light is incident upon the position detector surface 402. At this time, if the position of the substrate disk surface 403 changes among A, B, and C, the light path of the reflected light changes among A′, B′, and C′, respectively. The change in the light path of the reflected light is detected as a change in the position at which the light is incident upon the position detector surface 402, whereby the displacement of the substrate disk surface 403 can be detected.
However, in the conventional method as shown in FIG. 6, the change in the position of the reflected light on the position detector surface 402, which is detected as a signal, is caused not only by the displacement amount of the substrate disk surface 403 but also by the tilt of the substrate disk surface 403. The components caused by the tilt increase in proportion to the distance from a reflection point on the substrate disk surface 403 to the position detector surface 402. Accordingly, as compared with the components caused by the displacement, which remain constant independently of the distance from the reflection point to the position detector surface 402, the components caused by the tilt change at higher rates as the distance is increased. As described above, the conventional method is subject to the tilt as much as or more than the displacement of the substrate disk surface, and therefore, an accurate detection of a displacement is difficult.
In addition, in the above-described method, it is difficult to detect the focusing state of the electron beam on the substrate disk in real time during recording, and thus, the automatic focus control by feedback cannot be performed. As a result, a control based on the correspondence between the detected displacement of the surface of the substrate disk and the focus adjustment of the recording beam is required, and establishment of a particular technique for this control is desired.